The present invention generally relates to gimbaled thrusters and more particularly to a method and system for providing high-accuracy thrust trajectory tracking and solar array sun pointing during orbit transfer.
Gimbaled thrusters (including Hall Current Thrusters (HCTs)) are used by spacecraft, such as military communications spacecraft, to perform partial orbit transfer. The HCTs, which are much more fuel-efficient than chemical thrusters, enable as much as 500 kg of additional payload to be delivered to the mission orbit.
When used for orbit transfer, the HCTs are fired continuously for several months while the spacecraft attitude is controlled to track an optimal thrust trajectory vector. The attitude control system, which uses reaction wheel assemblies (RWAs) for attitude control and gimbaled HCTs for momentum adjust, maintains alignment of the HCT thrust axis with the trajectory vector. The system simultaneously rotates the spacecraft about the trajectory vector and positions the solar arrays to maintain accurate sun pointing. The sun pointing control is necessary to provide adequate electrical power to operate the spacecraft and the HCTs. The HCTs require roughly 9 kW. Trajectory tracking and sun pointing attitude control is straightforward except within orbital regions where the angular separation between the thrust trajectory vector and the sun vector is small. Within these regions, referred to as “proximity regions”, control torques and angular rates in excess of the control system's capability may be necessary for accurate sun pointing control.
Conventional methods address the foregoing problem simply by constraining the rate at which the spacecraft can rotate. While constraining the rotation rate may be effective, it can also create large solar array sun pointing errors which, in turn, may result in the need for a larger solar array or larger batteries with commensurate mass and cost penalties.
Hence, it would be desirable to provide a method and system that is able to provide more efficient sun pointing control in proximity regions.